Thermoelectric properties of a material may be expressed as ZT=TS2σ/k, where T is the absolute temperature, and S, σ, and K, respectively, refer to the Seebeck coefficient, the electrical conductivity and the thermal conductivity of the material. Inorganic semiconductors have been utilized in the fabrication of thermoelectric devices to convert heat into electricity due to their high electrical conductivity and large Seebeck coefficient. For example, an inorganic semiconductor was designed to have a ZT of about 2.4 from the use of Bi2Te3/Sb2Te3, whereby the improved thermoelectric performance was due to the reduced thermal conductivity and unchanged power factor (S2σ) of the Bi2Te3/Sb2Te3 compound. Furthermore, while tremendous research has been conducted to reduce thermal conductivities through the design of new complex materials, inorganic materials continue to suffer from large intrinsic thermal conductivities (>1 W/mK), which have severely limited the ability to develop improved inorganic thermoelectric devices.
However, organic thermoelectric devices, due to their low thermal conductivities (<0.5 W/mK), have emerged as a promising alternative to inorganic thermoelectric devices. In particular, among all organic semiconductor materials being used for thermoelectric devices, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate), referred to alternatively as PEDOT:PSS, has proven to be a desirable material for such thermoelectric devices. It was reported that the electrical conductivity of PEDOT:PSS thin films can be improved by three orders of magnitude via a single secondary dopant. For example, the electrical conductivity of a PEDOT:PSS thin film was improved from 0.06 S/cm to 298.52 S/cm after doping 5% (by volume) dimethyl sulfoxide (DMSO) into the PEDOT:PSS. It was also demonstrated that due to the strong interaction between DMSO and PSS, PEDOT in its polaron states were increased, which was responsible for improved electrical conductivity of the films. However, it has also been demonstrated that PEDOT in bipolaron states rather than in polaron states could achieve a larger Seebeck coefficient. Thus, it has been suggested that the undesirably low Seebeck coefficient of PEDOT:PSS thin films is due to the small amount of PEDOT in bipolaron states when the PEDOT:PSS films are doped with DMSO.
Therefore, there is a need for PEDOT:PSS thin films, doped with DMSO, which have increased PEDOT bipolaron states. In addition, there is a need for PEDOT:PSS thin films that have increased electrical conductivity, and an improved Seebeck coefficient, so as to enhance the thermoelectric properties of the PEDOT:PSS thin films. In addition, there is a need for an effective and simple process to enhance the thermoelectric properties, as well as to improve the electrical conductivity and Seebeck coefficient of PEDOT:PSS thin films.